Fluid detection device

ABSTRACT

One or a plurality of branch paths ( 3 ) having opening end portions ( 2 ) formed in the vicinity of an inner wall surface of a main path ( 1 ) through which a fluid flows so as to point the upstream or downstream side of the main path, and causing part of the fluid running in the vicinity of the inner wall surface of the main path to flow therethrough via the opening end portions, to thereby detect flow rates of the fluids running through the branch paths by using a thermal flow sensor ( 4 ). Preferably, a plurality of opening end portions ( 2   a ) pointed toward the upstream side or a plurality of opening end portions ( 2   b ) pointed toward the downstream side are arranged at regular intervals along the path cross section of the main path around the axis of the main path, to thereby measure a fluid flow with high accuracy.

TECHNICAL FIELD

The present invention relates to a fluid detecting device suitable formeasuring, for example, the flow rate of fuel gas or combustion air thatis supplied to a gas burner.

BACKGROUND ART

In recent years, low-NOx high-efficiency combustion, such asfully-premixed combustion, has been promoted, and it is required toaccurately control the supply amount of fuel gas and air (hereinafterreferred to as gas) to be supplied to, for example, a gas burner or agas engine. To achieve such control, it is important to detect the flowrate of the gas (that means fuel gas or combustion air) supplied to agas burner and the like with high accuracy.

For a flow detector for detecting the flow rate of gas (fluid), forexample, Unexamined Japanese Patent Application No. 4-230808 disclosesone in which a pair of temperature sensors arranged with/without aheater therebetween in the gas-flowing direction are provided onto theinner wall of a main path to be exposed. This thermal flowmeter detectstemperature distribution that is changed by the flow velocity of the gasby a temperature difference detected by the temperature sensors, tothereby measure the mass flow rate of the gas according to thetemperature difference. In the thermal flowmeter, however, a heater andthe temperature sensors are brought into direct contact with the gasrunning through the main path. Therefore, the thermal flowmeter is notsuitable for the flow rate measurement of the gas having a hightemperature of, for example about 300 degrees centigrade, in respect ofheat resistance. Moreover, in the thermal flowmeter, a detection outputwith respect to an increase in gas flow shows a curved alterationcharacteristic, so that an area in which the flow and the detectionoutput are considered to be in a proportional relation is narrow. Forthis reason, if the flow is to be detected in a relatively wide area,calculation for converting the detection output showing the curvedalteration characteristic with respect to the flow into a linearalteration characteristic is required.

Disclosed in, for example, Unexamined Japanese Patent Application No.10-307047 is an orifice-type flowmeter in which an orifice (throttle) isdisposed in a path, to thereby detect a gas flow rate by pressure(differential pressure) detected through the orifice. This orifice-typeflowmeter is so constructed as to shunt part of the fluid flowingthrough the main path into a branch path. Therefore, even ahigh-temperature gas can be detected after being refrigerated in thebranch path. On the other hand, in the orifice-type flowmeter, athrottle ratio of the gas path which is obtained by the orifice needs tofall in the range of from about 0.1 to 0.8, which is surely accompaniedby pressure loss.

Furthermore, it is necessary that an inflow-side opening end portion ofthe branch path be disposed on the upper stream side of the orifice, andan outflow-side opening end portion be disposed on the lower stream sideseparately. As a result, it is certain that the inflow-side opening endportion and the outflow-side opening end portion are positioned apartfrom each other at some distance in the longitudinal direction.Accordingly, in case that there generates oscillation in the gas flow inthe main path due to combustion or the like, the oscillationoccasionally cannot be detected by the orifice-type flowmeter. In otherwords, the orifice-type flowmeter has the disadvantage that oscillationat a specific frequency corresponding to the distance between theinflow-side opening end portion and the outflow-side opening end portioncannot be detected. This phenomenon is attributable to the fact that thepressure of the inflow-side opening end portion and that of theoutflow-side opening end portion are equalized, so that there generatesno flow in the branch path.

DISCLOSURE OF THE INVENTION

An object of the present invention consists in providing a fluiddetecting device that can be constructed such that even if fluid runningthrough a main path is high in temperature, a flow of the fluid can bedetected without the influence of the temperature thereof.

Another object of the invention is to provide a fluid detecting devicecapable of fully minimizing pressure loss and measuring a flow of afluid, such as fuel gas, with high accuracy with scarcely any influenceof the pressure loss.

Another object of the invention is to provide a fluid detecting devicecapable of setting a relatively wide area in which a flow and detectionoutput are considered to be in a proportional relation.

Another object of the invention is to provide a fluid detecting devicein which inflow-side opening end portions and outflow-side opening endportions of branch paths can be arranged close to each other, the devicebeing capable of accurately detecting oscillation of a gas flow,attributable to combustion.

In other words, the invention has been made to accomplish at least oneof the above objects.

To achieve the above objects, the invention has been made in light of:

-   (a) the fact that a fluid flow on a wall surface of a pipe can be    considered as a Couette flow when there is no throttle in a path    thereof; and-   (b) the fact that a flow velocity in the vicinity of an inner wall    surface of the pipe is in an approximately proportional relation    with an average flow rate in the pipe, although a flow velocity in    each part of a path cross section formed by the pipe through which    fuel gas or the like flows varies depending on a curved shape of the    pipe and distance from the wall surface of the pipe.

The fluid detecting device according to the invention is provided withone or a plurality of branch paths each having an opening end portionthat is formed in the vicinity of an inner wall surface of the main pathso as to point to an upstream or downstream side of the main path in thevicinity of the inner wall surface of the main path through which afluid runs, and causing part of a fluid flowing in the vicinity of theinner wall surface of the main path to run therethrough via the openingend portions, to thereby detect the flow rates of the fluids that runthrough the branch paths by using a thermal flow sensor.

With a fluid detecting device thus constructed, it is possible to shuntthe fluids into the branch paths without substantially providing athrottle in the main path. Thus, great pressure loss does not occur inthe main path. Even if the gas running through the main path is high intemperature, only a small amount thereof is shunted into the branchpaths and gives off heat onto the inner wall in the branch paths, thuslowering the gas temperature. This makes it possible to carry out flowdetection within heat resistance limits of the thermal flow sensor.

The device is particularly designed such that part of a laminar boundarylayer or of a laminar sub-layer flowing in the vicinity of the innerwall surface among the fluid running through the main path is shuntedinto the branch paths, to thereby detect the flows (part of the laminarboundary layer or of the laminar sub-layer) of the fluids that runthrough the branch paths. As a result, the area in which the fluid flowand the detection output are considered to be in a proportional relationis wide. It is possible to carry out detection with few error in thelaminar boundary layer or in the laminar sub-layer because these layersare hardly influenced by disorder that occurs in the fluid flowingthrough the main path.

Preferably, opening end portions pointed toward the upstream side of themain path or those pointed toward the downstream side of the main pathin the branch paths are arranged at regular intervals along a path crosssection around an axis of the main path, to thereby detect the flowrates of the fluids that run through the branch paths.

If the opening end portions of the branch paths are arranged at regularintervals along the path cross section in this manner, for example, evenin case that there generates deviation in the flow of the fluid runningthrough the main path due to the curve of the upstream-side path, thedeviation can be detected by difference in the flow rates detected inthe branch paths. It is also possible to measure an average flow rate ofthe fluid running through the main path, for example, by averaging theflow rates detected in the branch paths by arithmetic average.

It is also possible that the upstream-side branch paths whose openingend portions are pointed toward the upstream side and thedownstream-side branch paths whose opening end portions are pointedtoward the downstream side are jointed to each other via a communicatingportion that forms one path, and a total flow rate of the fluids runningthrough the branch paths is detected in a lump in the communicatingpath.

Since the above construction makes it possible to easily detect thetotal flow rates of the fluids running through the branch paths, forexample, even if there generates deviation in the flow of the fluid thatruns through the main path, it is possible to average the flow rates ofthe fluids that flow into the branch paths to perform the flow ratemeasurement. Consequently, the highly accurate flow rate measurement canbe easily carried out without the influence of deviation of the fluidthat runs through the main path.

Under the condition that the branch paths are left open at the other endsides, the fluids shunted from the main path into the branch paths maybe discharged outside, or to the contrary, the fluids that flow in fromthe outside via the branch paths may be flowed into the main path. Withthe above construction, part of the fluid that runs through the mainpath is discharged outside, or the fluids that flow in from the outsideare introduced into the main path. For example, if the fluid is air, noparticular problem occurs, and the construction can be simplified.

The device, however, is preferably constructed such that the opening endportions of the branch paths whose opening end portions are pointedtoward the upstream side are positioned upstream from the opening endportions of the branch paths whose opening end portions are pointedtoward the downstream side, and the other end portions of the branchpaths communicate with one another as mentioned, to thereby return thepart of the fluid shunted into the branch paths to the main path. Thisdoes not cause the problem that is produced in case that the branchpaths are open toward a surrounding environment at the other endsthereof, and the flow detection can be carried out while the flow itselfis stabilized.

Path resistance of each of the branch paths is made greater than pathresistance of the communicating portion, so that even if a flow in anyone of the opening end portions is locally changed, an influence on theflow as arithmetic average of the communicating portion can be maderelatively small. This minimizes the effects of deviation of flowdistribution in the main path.

It is also effective that in addition to the above-mentioned thermalflow sensor, an auxiliary thermal flow sensor having the samespecification as the above thermal flow sensor is disposed in a positionthat does not interfere with the fluids running through the branchpaths, and the output of the thermal flow sensor is converted by usingthe output of the auxiliary thermal flow sensor. The simultaneous use ofthe auxiliary thermal flow sensor makes it possible to, for example,counteract oscillation transmitted to the fluid and electrical noisestransmitted to the thermal flow sensor, thereby further heightening themeasurement accuracy.

“Path resistance” here means an approximate proportionality constantbetween the flow rate of the fluid running through a certain path anddifferential pressure between both ends of the path. For example, whenthe differential pressure is fixed, the flow rate is reduced byincreasing the path resistance. In general, the smaller the diameter ofthe path is, or the longer the path is, the greater the path resistancebecomes.

With the fluid detecting device according to the invention which is thusconstructed, since the fluid flow that is extremely close to the innerwall of the main path is shunted to detect the shunted flows, at leastone of the following advantages can be exhibited.

-   (1) The device can be constructed to be capable of detecting the    flow of the fluid running through the path even if the fluid is high    in temperature, and constructed such that the pressure loss is    extremely small.-   (2) The device can be constructed such that the area in which the    flow and the detection output are considered to be in a proportional    relation is relatively wide.-   (3) It is possible to dispose the inflow-side-opening end portions    and the respective outflow-side opening end portions of the branch    paths close to each other, which enables the accurate detection of    flow oscillation of the fluid (gas).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of distribution of a flow of fluid in a straight pathfor explaining a flow rate detection principle of a fluid detectingdevice according to the present invention;

FIG. 2 is a view of distribution of a flow of fluid in a curved path forexplaining a flow rate detection principle of the fluid detecting deviceaccording to the present invention;

FIG. 3 is a cross-sectional view of a general construction of asubstantial part for explaining the basic construction of the fluiddetecting device according to an embodiment of the present invention;

FIG. 4 is a view showing a fixing position of an opening end portion ofa branch path in a cross section of a main path for explaining anotherembodiment of the fluid detecting device according to the presentinvention;

FIG. 5 is a pattern diagram of a structure of the branch path forexplaining further another embodiment of the fluid detecting deviceaccording to the present invention;

FIG. 6 is a view of a general construction of a substantial part of apreferred embodiment of the fluid detecting device according to thepresent invention;

FIG. 7 is a longitudinal sectional view of a general construction of thespecific fluid detecting device according to the present invention;

FIG. 8 is a cross-sectional view, taken along line X-X in the fluiddetecting device of FIG. 7;

FIG. 9 is a view of measurement characteristics of the fluid detectingdevice of FIGS. 7 and 8;

FIG. 10 is a view of a modification example of the fluid detectingdevice according to the present invention;

FIG. 11 is a perspective view of the outer appearance of an example of athermal flowmeter that is installed in the fluid detecting deviceaccording to the present invention;

FIG. 12 is a view of a basic circuit construction of the thermalflowmeter of FIG. 11;

FIG. 13 is a view of a general construction of a substantial part of thefluid detecting device according to further another embodiment of thepresent invention; and

FIG. 14 is a view of a general construction of an output conversioncircuit in the fluid detecting device of FIG. 13.

BEST MODE OF CARRYING OUT THE INVENTION

A fluid detecting device according to an embodiment of the presentinvention will be described below in detail with reference to drawings.

A flow of a fluid running through a pipe whose path cross section isformed in a circular shape is not uniform in each part of the path crosssection, and has a certain distribution as shown in FIGS. 1 and 2. To beconcrete, due to viscosity of the fluid, the flow is generally rapid ina central portion of the path cross section, and is slow in the vicinityof an inner wall surface of the pipe. Particularly in case that theupstream-side pipe is curved as shown in FIG. 2, the flow is distorteddue to the curve of the path, so that there generates deviation in theflow distribution.

A flow velocity in the vicinity of a wall surface of the pipe can bedenoted by an equation of a laminar boundary layer shown by aNavier-Stokes equation and a continuity equation. Without a throttle inthe path, a flow Δu in a position at a distance d(d≠0) from the wallsurface can be considered as a Couette flow, where the flow velocity inthe vicinity of the inner wall surface of the pipe is [0]. If a flowvelocity in the wall surface (y=0) is [u=0], and a velocity in aboundary condition (y=R) of the Couette flow is [u=U (average flowvelocity)], a flow velocity u thereof can be approximate as [u=U/R·y].

An average flow velocity uave in the vicinity of the inner wall surface(d≈0) in case that there is deviation in the fluid flow as shown in FIG.2 can be found as arithmetic average of a flow velocity U_(f) in thevicinity of the inner wall surface in circumferential parts along theinner wall surface that defines the path cross section, and thefollowing relation is established.U _(ave) ≈Σu _(f) /n=UΣd/R _(f) /nwhere Σ represents a summing calculation when a subscript f is (f=1, 2,. . . n). Therefore, the average flow velocity U can be expressed asfollows:U=(Σu _(f) /n)/(Σd/R _(f))Consequently, distance R_(f) from the inner wall surface to a positionwhere the fluid flows at the average flow velocity U is fixed. If a flowvelocity measurement position d is a fixed position within a range ofthe distance R_(f), the flow velocity measured at the measurementposition d and the average flow velocity U have a proportional relation.

The above observation result shows that the flow velocity in thevicinity of the inner wall surface of the pipe d is proportional to theflow rate in the pipe. When there is deviation in the flow, if the flowvelocity u_(f) in the vicinity of the inner wall surface is measured ina plurality of places where the pipe is divided into n equal parts alongthe path cross section, and the measured values are arithmeticallyaveraged, the average flow velocity U can be firmly found as follows:U=kΣu _(f) /nwhere k is a constant number. In short, the average flow velocity U canbe measured regardless of the deviation of the flow. The same is truewith the case that a turbulent flow occurs in the mainstream.

The invention has been made in light of the above knowledge and isbasically constructed to measure the flow velocity of the fluid in thevicinity of the inner wall surface of the main path formed in thecircular shape in path cross section.

FIG. 3 is a cross-sectional view of a general construction of asubstantial part of the fluid detecting device according to anembodiment of the invention. Reference numeral 1 represents a main path(circular pipe) formed in a circular shape in path cross section. Themain path 1 has a diameter that is fixed at least within an areaillustrated. The fluid detecting device includes at least one branchpath 3 having opening end portions (2 a, 2 b) in the vicinity of aninner wall surface 1 a of a pipe forming the main path 1, and shuntingand running part of the fluid that flows in the vicinity of the innerwall surface of the main path 1. The branch path 3 has a diameter thatis fixed at least within an area illustrated. The branch path 3 includesan upstream-side branch path 3 a whose opening end portion 2 a ispointed toward the upstream side of the main path 1 and/or adownstream-side branch path 3 b whose opening end portion 2 b is pointedtoward the downstream side of the main path 1. The fluid detectingdevice may be provided with both the upstream-side branch path 3 a whoseopening end portion 2 a is pointed toward the upstream side and thedownstream-side branch path 3 b whose opening end portion 2 b is pointedtoward the downstream side, but may be provided with either one of thebranch paths.

The opening end portions 2 of the branch path 3 are designed to have aheight that falls in the range of from, for example, about 0.3 to 1.0 mmfrom the inner wall surface 1 a of the pipe so that an opening position(measurement position d) thereof is closer to the inner wall surface ofthe pipe than the boundary R_(f) of the Couette flow in order to shuntonly the Couette flow in the vicinity of the inner wall surface 1 a ofthe pipe into the branch path 3. For convenience, distance that extendsfrom the inner wall surface of the pipe toward the center of the pipe iscalled “height”. To be more specific, the opening end portions 2 eachhave a bore with a diameter of 0.8 mm, and the center of the bore ispositioned at a height of 0.5 mm from the inner wall surface 1 a of thepipe. The opening end portions 2 disposed at such a height shunt part ofthe flow of the fluid running through the main path, a so-called laminarboundary layer or laminar sub-layer, into the branch path 3.

The flow velocities (flow rates) of the fluids running through thebranch path 3 (3 a, 3 b) are detected by means of thermal flowmeters 4(4 a, 4 b) installed in the branch path 3. Detection signals of thethermal flowmeters 4 are transmitted to a signal processing portion 5constructed of a microprocessor and the like, to thereby measure theaverage flow velocity (flow rate) U of the fluid running through themain path 1. Such thermal flowmeters 4 are well known as disclosed infor example the above-mentioned Unexamined Japanese Patent ApplicationNo. 4-230808.

In case that the branch path 3 is open toward a surrounding environment(the atmosphere, for example) at the other end, the part of the flowwhich is shunted from the main path 1 via the inflow-side opening endportion 2 a is discharged outside via the branch path 3 a. The fluidthat flows from the outside into the branch path 3 b is introduced fromthe outflow-side opening end portion 2 b into the main path 1. In eithercase, the flow in the vicinity of the inner wall surface 1 a of the pipewhich is provided with the opening end portions 2 is a laminar boundarylayer or laminar sub-layer that flows at a uniform speed in a pathdirection. The flows of the fluids running through the branch paths 3 a,3 b depend on the flow of the laminar boundary layer or laminarsub-layer.

The branch paths 3 a, 3 b virtually shunt and run part of the laminarboundary layer or laminar sub-layer, and only difference between thebranch paths 3 a, 3 b is that one of them shunts part of the flow of thelaminar boundary layer or laminar sub-layer in the main path 1 while theother runs the fluid added from the outside as part of the flow of thelaminar boundary layer or laminar sub-layer. Therefore, the flowvelocity of the laminar boundary layer or laminar sub-layer can beaccurately detected by detecting the flow velocities of the fluids thatrun through the branch path 3 (3 a, 3 b).

A cross-sectional area of the main path 1 in the vicinity of the openingend portions 2 is fixed all the time, so that the product of thecross-sectional area and the flow velocity is corresponding(proportional) to the flow rate. For this reason, the terms “flowvelocity” and “flow rate” are occasionally used here as replaceable witheach other. In addition, they are generically called “flow”.

The branch path 3 (3 a, 3 b) may be plurally arranged along thecircumference of the inner wall surface 1 a of the pipe in the pathcross section formed by the main path 1. In this case, the opening endportions 2 of the branch path 3 are disposed at equiangular intervalsaround an axis of the main path 1 as illustrated in FIG. 4 showing across section of the main path 1 as a pattern diagram. Specifically, incase that four branch paths 3 are provided, the inner wall surface 1 aof the pipe of the main path, which is in an annular shape, is dividedinto equal parts at an angle of 90 degrees, and the opening end portions2 of the branch paths 3 are located in the equally-dividing positions.In this case, it is desired that the branch paths 3 have the samediameters (for example, 0.8 mm in bore size). By detecting the flowvelocities of the fluids shunted into the respective branch paths 3, forexample even if there is deviation in the flow of the fluid runningthrough the main path 1 as illustrated in FIG. 2, it is possible todetect the flow velocity in each part of the inner wall surface 1 a ofthe pipe according to the deviation.

Consequently, if the flow velocities in the respective parts of theinner wall surface 1 a of the pipe which are detected through the branchpaths 3 are comprehensively judged, and for example, difference of theflow velocities in the respective parts are found, it is possible toevaluate the deviation in the flow of the fluid running through the mainpath 1. Furthermore, if the flow velocities in the respective parts ofthe inner wall surface 1 a of the pipe which are detected through thebranch paths 3 are arithmetically averaged, the average flow rate can befound without difficulty, regardless of the deviation in the flow. Inother words, even if there is a curved pipe line on the upstream side ofa flow rate measurement portion, and as a result, there is distortion(deviation of the flow velocity distribution) in the flow of the fluidrunning through the main path 1, the average flow velocity can bemeasured with ease and accuracy.

Again, the opening end portions 2 arranged at equiangular intervalsaround the axis of the main path 1 may be either the inflow-side openingend portion 2 a pointed toward the upstream side of the main path 1 orthe outflow-side opening end portion 2 b pointed toward the downstreamside of the main path 1. The opening end portions 2 may also be disposedat a fixed height that falls in the range of from, for example, about0.3 to 1.0 mm from the inner wall surface 1 a of the pipe such that theopening positions (measurement positions d) are lower than the boundaryR_(f) of the Couette flow.

The branch paths 3 may be connected in parallel with each other througha connecting path 6 for example as illustrated in FIG. 5, and thethermal flowmeter 4 may be provided to the connecting path 6 thatconverges the fluids running through the branch paths 3. That is to say,the other end portions of the branch paths 3 provided with the openingend portions 2 in positions where the main path 1 is divided into equalparts in the circumferential direction are caused to communicate withthe connecting path 6 formed in an annular shape along the outercircumference of the main path 1. The thermal flowmeter 4 may beinstalled in a converging path 6 a branching from a part of theconnecting path 6.

In this case, the connecting path 6 and the converging path 6 acorrespond to the communicating portion. For a desired embodiment, pathresistance of at least either the branch paths 3 or the connecting path6 should be set to be great, and path resistance of the converging path6 a to be small. To this end, for example, path cross-sectional area ofeach of the branch paths 3 is set to be 0.5 mm², and pathcross-sectional area of the connecting path 6 is set to be 4.0 mm² whichis greater than the total of the path cross-sectional areas of the fourbranch paths 3, which is 2.0 mm², and path cross-sectional area of theconverging path 6 a is set to be 6.0 mm² which is still greater. By sodoing, the path resistances can get smaller in order from the branchpaths 3 to the connecting path 6 to the converging path 6 a. However, ifthe path cross-sectional area of the converging path 6 a is greatenedtoo much, the flow velocity becomes too small and gets out of adetection range of the thermal flowmeter 4, so that attention isdemanded.

If the fluid detecting device is thus constructed, part of the fluidflow shunted in the vicinity of the wall surface of the main path 1 inthe opening end portions 2 flows through the branch paths 3 into theconnecting path 6 to be converged into one flow in the converging path 6a. The thermal flowmeter 4 therefore detects a total flow ratecorresponding to the total of the flows shunted into the branch paths 3.If the total flow rate detected by the thermal flowmeter 4 is divided bythe number n of the branch paths 3, a flow rate for one branch path 3,namely an average flow rate of the branch paths 3, can be calculated.Furthermore, it is possible to easily measure an average flow velocityof the fluid (laminar boundary layer or laminar sub-layer) that runs inthe vicinity of the inner wall surface of the main path 1, and then bymultiplying the average flow velocity by a prescribed constant number,it is possible to measure an average flow velocity of the fluid runningthrough the main path 1.

To employ the above construction eliminates the need for installing thethermal flowmeters 4 in the respective branch paths 3, and only onethermal flowmeter 4 will be sufficient. As a consequence, for example,in spite of the need for the connecting path 6 for the parallelconnection of the branch paths 3, the entire construction including thesignal processing system can be simplified. Again, the branch paths 3for parallel connection may be either the branch paths 3 a whose openingend portions 2 a are pointed toward the upstream side of the main path 1or the branch paths 3 b whose opening end portions 2 b are pointedtoward the downstream side of the main path 1.

In case that the main path 1 is provided with the upstream-side branchpaths 3 a whose opening end portions 2 a are pointed toward the upstreamside of the main path 1 and the downstream-side branch paths 3 b whoseopening end portions 2 b are pointed toward the downstream side of themain path 1, the other end portions of the branch paths 3 a, 3 b may bemade to communicate with each other as illustrated in FIG. 6, to therebyreturn the part of the fluid, which is introduced from the main path 1into the upstream-side branch path 3 a, to the main path 1 via thedownstream-side branch path 3 b.

In this case, the opening end portions 2 a, 2 b of the branch paths 3 a,3 b preferably face in opposite directions so that the fluid shuntedinto the branch path 3 a is not returned upstream from the opening endportion 2 a via the branch path 3 b. In other words, the fluids shuntedinto the branch paths 3 a, 3 b are preferably returned to branch pointsor downstream therefrom. However, the possibility that the fluid shuntedinto the branch path 3 a is returned upstream from the opening endportion 2 a via the branch path 3 b is not denied.

In case that the branch paths 3 a whose opening end portions 2 a arepointed toward the upstream side of the main path 1 and the branch paths3 b whose opening end portions 2 b are pointed toward the downstreamside of the main path 1 are provided in pairs, the other end portions ofthe branch paths 3 a, 3 b may be individually made to communicate witheach other. As stated above, however, if the branch paths 3 a and therespective branch paths 3 b are arranged to be connected in parallel,the other end portions (the other end portion of the connecting path 6)of the branch paths 3 a, 3 b that are connected in parallel are made tocommunicate with each other.

If the other end portions of the branch paths 3 a whose opening endportions 2 a are pointed toward the upstream side of the main path 1 andthe other end portions of the branch paths 3 b whose opening endportions 2 b are pointed toward the downstream side of the main path 1are made to communicate with each other as described, part of the fluidsshunted from main path 1 into the branch paths 3 a are returned to themain path 1 via the branch paths 3 b, which makes it possible tostabilize the flows thereof. Moreover, the fluid running through themain path 1 is not discharged outside thereof, and the fluid introducedfrom the outside does not get mixed with the fluid running through themain path 1, so that there generates no change in the fluid itself whichruns through the main path 1. This makes it possible to detect the flowvelocity (flow rate) of the fluid running through the main path 1without influencing the fluid. In addition, operation reliability can befully improved as a fluid detecting device.

The fluid detecting device thus constructed can be embodied as, forexample, a low-profile device as illustrated in FIGS. 7 and 8. FIG. 7shows a structure of a longitudinal section of the fluid detectingdevice, and FIG. 8 shows a structure of a cross section of the fluiddetecting device, taken along a broken line X-X in FIG. 7. The upstreamside is on the right, and the downstream side on the left, facing intothe drawing as viewed

The device is so constructed as to be applied in a state interposed in aflange coupling portion of a cylindrical gas pipe through which fuel gasor the like runs. The device includes a ring-shaped first member 10having prescribed thickness (9 mm, for example) and a ring-shaped secondmember 20 fitted to an outer circumference of the first member 10. Thefirst and second members 10, 20 are formed of metal members, but may bemade of plastic having high heat resistance.

An annular inner wall surface 11 of the ring-shaped first member 10having the prescribed thickness forms the main path 1 for the fluid. Theinner wall surface 11 has an internal diameter D that is varied to besmoothly curved in a thickness direction of the first member 10(fluid-flowing direction) to be formed into a curved surface having amoderately streamlined shape forming a semicircular arc in section or asemi-ellipse in section in which the internal diameter D is minimum inthe central portion.

In consideration of the internal diameter (21 mm, for example) of thegas pipe, the maximum internal diameter D of the inner wall surface 11is set to be 21 mm in both end portions. Inward projection height h inthe central portion of the inner wall surface is set to fall in therange of from, for example, about 0.5 to 1.0 mm. More specifically, theminimum internal diameter D-2 h of the inner wall surface 11 is set tofall in the range of from about 19 to 18 mm, thus making pressure lossgiven to the fluid running through the main path extremely small inconsort with the above-mentioned smoothly curved surface.

In the first member 10, a plurality of through-holes 12 that define thebranch paths 3 provided with the opening end portions 2 are bored fromthe inner wall surface 11 to an outer circumferential surface 14 a ofthe first member 10. The opening end portions 2 are each formed into anelliptical recessed portion that opens in the thickness direction of thefirst member 10 to have a funnel-like shape from the central portion ofthe inner wall surface 11 having a semicircular arc-shaped cross sectionor a semi-elliptical-shaped cross section through both side portionsthereof. The opening end portions 2 formed into the recessed portionsare each disposed to open at an angle of, for example, substantially 60degrees with respect to the axis of the main path 1. In FIG. 7, theopening end portions 2 a and the respective opening end portions 2 b aredrawn slightly apart from each other from side to side. When the fluiddetecting device is actually designed, however, the opening end portions2 a and the respective opening end portions 2 b may be completelysuperimposed upon each other in the cross-sectional direction byoptimizing the angle.

One of the opening end portions 2 arranged from the central portion ofthe inner wall surface 11 through both the side portions functions asthe inflow-side opening end portion 2 a that opens toward the upstreamside of a fluid-flowing direction, and the other as the outflow-sideopening end portion 2 b that opens toward the downstream side of thefluid-flowing direction. The opening end portions 2 (2 a, 2 b) pointedtoward the upstream and the downstream side in the fluid-flowingdirection are disposed in positions dividing the inner wall surface 11into equal parts in the circumferential direction as described above,thereby shunting and introducing part of the flow of the laminarboundary layer or laminar sub-layer which runs in the vicinity of theinner wall surface 11 of the first member 10 into the through-holes 12(branch paths 3).

By employing the above construction, the inflow-side opening endportions 2 a and the respective outflow-side opening end portions 2 b ofthe branch paths are positioned extremely close to each other, whichvirtually resolves the problem that sensitivity is decreased withrespect to a certain oscillation frequency (wavelength) of the fluid.

Two grooves 13 a, 13 b are formed in an outer circumferential surface 14b of the first member 10 in parallel with each other in thecircumferential direction. The grooves 13 a, 13 b communicate with theother end portions of the through-holes 12 at bottom portions thereof(namely, the outer circumferential surface 14 a). Upper surfaces of thegrooves 13 a, 13 b are closed by an inner circumferential surface 23 ofthe second member 20 which is fitted to the outer circumferentialsurface of the first member 10 to form the annular communicating path 6.

The second member 20 includes the inner circumferential surface 23 thatis fitted to the outer circumferential surface of the first member 10 toclose the upper surfaces of the grooves 13 a, 13 b as mentioned and arecessed space portion 21 that is pierced in part of the innercircumferential surface 23 to cause the grooves 13 a, 13 b tocommunicate with each other. A flowmeter-fixing hole having a prescribedshape is pierced in a bottom surface of the space portion 21. Thethermal flowmeter 4 mounted on a package having a prescribed shape isinterfitted in the flowmeter-fixing hole from a outer circumferentialsurface side of the second member 20.

In a state where the upper surfaces of the grooves 13 a, 13 b are closedand fit together, that is, in a state where the first member 10 isinterfitted with the inner circumferential surface 23 of the secondmember 20, the first member 10 and the second member 20 are coupled andintegrated with each other, for example, by tightly welding the firstmember 10 and the second member 20, or the like. Such integration causesthe branch paths 3 defined by the through-holes 21 to communicate withthe connecting path 6 defined by the grooves 13 a, 13 b via the spaceportion 21. The cross-sectional area of each of the branch paths 3 isdetermined such that the narrowest portion of the opening end portion 2is the smallest, and so as to get larger in the grooves 13 and the spaceportion 21 in order of mention. As a consequence, fluid resistance isset to get smaller from the grooves 13 to the space portion 21.

Although not shown for simplification, in FIG. 7, an annular groove maybe formed in a contact surface of the outer circumference of the firstmember 10 and the inner circumference of the second member 20 betweenthe grooves 13 a and the grooves 13 b, and a gasket, such as an O ring,may be disposed in the annular groove. Such a gasket prevents theleakage of the fluids that run from the grooves 13 a through a gap ofthe contact surface into the grooves 13 b, the leakage beingattributable to a deficiency in operating accuracy. As a result, thefluids running through the grooves 13 a are sure to flow into thegrooves 13 b via the space portion 21, which promotes the prevention ofdetection errors.

In the second member 20 and gaskets 30, 30 for maintaining airtightness,a plurality of circular holes 22 are formed in equiangular intervals asillustrated in FIG. 8 so as to avoid where the space portion 21 isdisposed. The circular holes 22 are for inserting bolts, not shown,which are to be fitted to the flange coupling portion of a gas pipe tocouple a pair of opposite flange portions. In place of the gaskets 30,an annular groove may be formed in a surface opposite to the pipe flangein the second member 20, and an O-ring-shaped gasket may be situatedtherein. A material for the gaskets may be suitably selected fromwell-known materials, such as rubber and soft copper, according toworking temperature. If the gaskets 30, 30 is made of a material havinglow heat conductivity (for example, a mold of heat-resistant fibers),and the second member 20 is made of a material having high heatconductivity (for example, aluminum), the shunted combustion gas can beefficiently refrigerated. In order to further encourage therefrigeration of the combustion gas, a path length of the space portion21 may be elongated by increasing the diameter of the second member 20,or a cooling fin or a cooling system for refrigerating a surroundingarea of the space portion 21 may be situated outside.

The low-profile fluid detecting device having the above construction canbe easily installed in a fuel gas supply system, such as a gas turbine,if only sandwiched between the flange coupling portions of an existinggas pipe to be interposed in the path. Additionally, part of the flow ofthe laminar boundary layer or laminar sub-layer of the fuel gas runningthrough the gas pipe, which runs in the vicinity of the wall surface ofthe gas pipe, is introduced to the space portion 21, to therebyeffectively measure the flow velocity (flow rate). Since the flowvelocity (flow rate) of the fuel gas running through the gas pipe can bemeasured as flow velocities of the fluids flowing through the branchpaths 3 with high accuracy, practical advantages of the device areenormous.

Even if the fuel gas running through the gas pipe is high intemperature, or if the fuel gas contains dust, since the measurement isconducted after bypassing part of the fuel gas via the branch paths 3,the fuel gas can be refrigerated halfway, and furthermore the dust canbe removed by an inertial dust collection effect possessed by thebending path structure. In general, even the thermal flowmeter 4 thatdoes not have very high heat resistance can perform the measurement withease and stability. Unlike a flowmeter that carries out flow measurementby using differential pressure produced by a conventional orifice, thethermal flowmeter 4 detects a mass flow rate of the fluid by the flow inthe vicinity of the inner wall of the path, thereby bringing advantagesthat the measurement can be performed with high accuracy without causingthe problem of pressure loss and the like.

FIG. 9 shows a relation between a flow rate measured by using the fluiddetecting device with the above structure and an actual flow rate of thefluid running through the main path 1 in comparison. A characteristic Ain FIG. 9 shows a measurement characteristic in case that straight pipes(gas pipes) each having an internal diameter of 21 mm are connected tothe upstream and the downstream side, respectively, via flanges. Acharacteristic B shows a measurement characteristic in case thatstraight pipes (gas pipes) each having an internal diameter of 27 mm areconnected to the upstream and the downstream side, respectively, viaflanges. A characteristic C shows a measurement characteristic in casethat a straight pipe (gas pipe) having an internal diameter of 21 mm isconnected to the upstream side, and a straight pipe (gas pipe) having aninternal diameter of 27 mm to the downstream side.

A characteristic D shows a measurement characteristic in case that abending pipe (gas pipe) having an internal diameter of 21 mm isconnected to the upstream side, and a straight pipe (gas pipe) having aninternal diameter of 21 mm to the downstream side. A characteristic Eshows a measurement characteristic in case that a bending pipe (gaspipe) having an internal diameter of 27 mm is connected to the upstreamside, and a straight pipe (gas pipe) having an internal diameter of 27mm to the downstream side. Lastly, a characteristic F shows ameasurement characteristic in case that a bending pipe (gas pipe) havingan internal diameter of 21 mm is connected to the upstream side, and astraight pipe (gas pipe) having an internal diameter of 27 mm to thedownstream side.

As shown by the characteristics A through F, it was verified that thefluid detecting device according to the invention made it possible toobtain detection characteristics shown by substantially straight lines,while remaining nearly unaffected by the internal diameter of the gaspipe, and despite whether the gas pipe was a straight or bending pipe,that is to say, regardless of presence or absence of distortion of thefluid flow. It was also verified that there was little error between amain flow rate and an average sidewall flow rate. In other words, thearea in which the flow and the detection output were considered to be ina proportional relation was relatively wide, so that it was verifiedthat sufficient detection accuracy is attained.

In light of these experimental results, for example as shown in FIG. 10,even if the opening end portions 2 are formed in tip ends of portions 7which narrow the path cross-sectional area of the main path 1 andprotrude in the inside of the pipe, the protrusion height thereof doesnot cause a big problem as long as the protruding portions 7 do nothamper the laminar boundary layer or the laminar sub-layer that flows inthe vicinity of the inner wall surface of the main path 1. Consideringthat a standard gas pipe has an internal diameter of 21 mm or 27 mm, ifonly there is provided the fluid detecting device in which a minimuminternal diameter of the main path 1 is 19 mm, the device can be appliedto a standard pipe having a bore diameter of 21 mm or 27 mm withoutchange.

The thermal flowmeter 4 installed in the flow rate-detecting device willbe briefly described below. The thermal flowmeter has an elementalstructure in which a pair of temperature sensors Ru, Rd made up ofresistance thermometer bulbs are arranged in a fluid-flowing direction Fsuch that a heater element Rh made up of a heating resistive elementdisposed on a silicon base B is interposed therebetween, for example asshown in FIG. 11, and the flowmeter is called a micro flow sensor. Thethermal flowmeter (micro flow sensor) detects a flow rate Q of the flowby a resistance value change attributable to heat of the temperaturesensors Ru, Rd, using the fact that a diffusion level (temperaturedistribution) of heat that is generated by the heater element Rh ischanged according to the fluid flow.

To be concrete, the thermal flowmeter measures the flow rate Q, usingthe fact that the heat generated by the heater element Rh is added tothe downstream-side temperature sensor Rd according to the flow rate Qof the fluid, and this makes the resistance value change attributable tothe heat of the temperature sensor Rd greater than the upstream-sidetemperature sensor Ru. Reference character Rr in the drawing representsa temperature sensor made up of a resistance thermometer bulb disposedaway from the heater element Rh and is used for measurement of ambienttemperature.

FIG. 12 shows a general construction of a thermal flowmeter using theabove-mentioned micro flow sensor. A drive circuit of the heater elementRh is constructed such that a bridge circuit 41 is formed by using theheater element Rh, the temperature sensor Rr for ambient temperaturemeasurement, and a pair of fixed resistors R1, R2, to thereby applyvoltage Vcc supplied from a prescribed power source to the bridgecircuit 41 through a transistor Q. Simultaneously, bridge output voltageof the bridge circuit 41 is found by a differential amplifier 42, andthe transistor Q is subjected to feedback control so that the bridgeoutput voltage becomes zero, to thereby adjust heater drive voltageadded to the bridge circuit 41. Due to the heater drive circuit thusconstructed, heating temperature of the heater element Rh is socontrolled as to be constantly higher than ambient temperature t only bya constant temperature difference Δt.

A flow rate detecting circuit for detecting the flow rate Q of the fluidrunning along the micro flow sensor by a resistance value changeattributable to the heat of the temperature sensors Ru, Rd isconstructed such that a bridge circuit 43 for flow rate measurement isformed by using the temperature sensors Ru, Rd and a pair of fixedresistors Rx, Ry, to thereby detect bridge output voltage according tothe resistance value change of the temperature sensors Ru, Rd by meansof a differential amplifier 44. Under the condition that a heating valueof the heater element Rh be constant by using the heater drive circuit,the flow rate Q of the fluid running along the micro flow sensor isfound by bridge output voltage Vout detected by the differentialamplifier 44. A characteristic of such a micro flow sensor is thecapability of measuring an air current of an extremely low flow velocity(for example, 0.3 mm/second at a lower limit). Consequently, even if theflow rates of the branch paths are extremely small (for example, about1/1000) in relation to the flow rate of the main path as in the aboveembodiment, the fluids running through the branch paths can be detectedwith high sensitivity.

In the thermal flowmeter thus constructed, if heater drive voltage Vhapplied to the bridge circuit 41 is monitored as illustrated in FIG. 12,since the heater drive voltage Vh corresponds to a change in the ambienttemperature, for example, it is possible to consider a temperaturechange attributable to combustion oscillation transmitted from acombustion chamber as a rate of change (amplitude) and oscillationfrequency of the heater drive voltage Vh. In other words, the combustionoscillation is transmitted to the fuel gas in the gas pipe or the like,which is near the combustion chamber, and ambient air. Therefore, ifsuch combustion oscillation is considered as a change in the drivevoltage of the heater element Rh in the thermal flowmeter, this makes itpossible to monitor the combustion oscillation without difficulty. It isthen possible to perform the flow rate measurement using the thermalflowmeter and to monitor the combustion oscillation using the samethermal flowmeter.

In case that the thermal flowmeter is realized, for example asillustrated in FIG. 13 showing a general construction of a substantialpart thereof, there may be provided an auxiliary space portion 25 awayfrom the branch paths 3 in addition to the space portion 21 that linksthe connecting path 6 made up of the grooves 13 a, 13 b to form thecommunicating path, and an auxiliary thermal flowmeter (flow ratesensor) 4 a may be disposed in the auxiliary space portion 25. Theauxiliary space portion 25 is a airtight space created by closing anopening portion of a recessed portion with a bottom which is bored inthe outer circumferential surface of the second member 20 with theauxiliary thermal flowmeter (flow rate sensor) 4 a fixed to the secondmember 20. In short, the auxiliary space portion 25 is formed as a spacethat does not interfere with the fluid flow. The auxiliary thermalflowmeter 4 a that is specified similarly to the thermal flowmeter 4 isdisposed, to thereby detect a state of the flow in the inside of theauxiliary space 25 irrelevant to the fluid flow.

Applied to the fluid in the inside of the auxiliary space portion 25 isoscillation relatively similar to the oscillation applied to the fluidsrunning through the branch paths 3, along with automatic oscillationgenerated in the pipe to which the fluid detecting device is fixed. Thethermal flowmeter (flow rate sensor) 4 and the auxiliary thermalflowmeter (flow rate sensor) 4 a are applied with basically similarnoises under electrical and magnetic influences from the outside. As aconsequence, detected by the auxiliary thermal flowmeter 4 a disposed inthe auxiliary space portion 25 are only external factors (oscillationand noises) applied to the fluid in the auxiliary space 25 and internalnoises produced in the auxiliary thermal flowmeter 4 a. The use of theauxiliary thermal flowmeter 4 a makes it possible to detect elementsincluding the oscillation, electrical noises, and the like, transmittedfrom a combustion chamber and the like through the pipe. The thermalflowmeter 4 and the auxiliary thermal flowmeter 4 a are each constructedas illustrated in FIG. 12.

As illustrated in FIG. 14, a signal element Vout(N) that does notinterfere with the fluid flow which is detected by the auxiliary thermalflowmeter 4 a is subtracted from the output Vout corresponding to thefluid flow (flow rate) which is detected by the thermal flowmeter 4. Inthis case, it is desirable that after the signal element Vout(N) ismultiplied by a prescribed conversion coefficient K, the result be usedfor conversion of the output Vout of the thermal flowmeter 4. That is tosay, the output Vout corresponding to the fluid flow (flow rate) whichis detected by the thermal flowmeter 4 includes the signal elementVout(N) that is relatively detected by the auxiliary thermal flowmeter 4a. For this reason, if the output Vout(N) of the auxiliary thermalflowmeter 4 a is subtracted from the output Vout of the thermalflowmeter 4, it is possible to measure only an element (flow rate)dependent on the fluid flow with high accuracy.

Therefore, if the signal element Vout(N) included in the fluid andindependent of the fluid flow is detected by the auxiliary thermalflowmeter 4 a apart from the thermal flowmeter 4, and the output Vout ofthe thermal flowmeter 4 is converted using the signal element Vout(N),it is possible to effectively drown out noise elements included in thefluid flow, attributable to oscillation and the like, the electricalnoises produced by the thermal flowmeter 4 and the auxiliary thermalflowmeter 4 a themselves, and the like. Therefore, highly accurate flow(flow rate) measurement can be carried out with a higher signal-to-noiseratio.

Depending on the volume of the auxiliary space portion 25, however,there is a possibility that the auxiliary space portion 25 functions asan automatic filter with respect to oscillation. In order to resolvesuch a problem, for example, the auxiliary space portion 25 may befilled with atmosphere by forming a small hole, not shown, from theouter circumferential surface of the second member 20 to the auxiliaryspace portion 25. Alternatively, the auxiliary space portion 25 may befilled with the measured fluid by forming a small hole, not shown, fromthe inner circumferential surface 23 of the second member 20 to theauxiliary space portion 25. This should be done with care so that theauxiliary space portion 25 does not hamper the fluid flow. In the caseof the structure in which the auxiliary space portion 25 is open to theatmosphere or the measured fluid as mentioned above, dimensions and ashape of the auxiliary space portion 25 and of the small hole should beproperly adjusted so that oscillation elements in a measurementfrequency band are not decreased.

The present invention is not limited to the above embodiment. Forexample, the number n of the opening end portions 2 that are plurallyformed along the inner wall surface of the main path 1 in thecircumferential direction thereof may be determined in accordance withthe specification thereof. As to the size and shape of the opening endportions 2, modification can be made in various ways. For example, inplace of the recessed portion, a tubular protruding portion may beprovided. The sectional shape of the main path 1 is not limited to acircle. When n opening end portions 2 are formed in the main path 1having a shape other than the circular shape, an optimum installationlocation for each of the opening end portions 2 is preferably found outthrough an experiment. Alternatively, the sectional area of the mainpath 1 may be divided into n equal parts along virtual lines (where thevirtual lines pass through the center of the cross section of the mainpath), to thereby form the opening end portions 2 on the virtual centerlines of the respective divided pieces. This corresponds to thedisposition of the opening end portions 2 at equal angles around themain path 1 when the main path 1 has a circular cross section. As to thethermal flowmeter, in place of the micro flow sensor integrated on asemiconductor, a flowmeter with a temperature sensor constructed of athermopile (thermocouple) or a thermal flowmeter using a wire of severalmicrons may be utilized. The point is that various modifications can bemade without deviating from the gist of the invention.

1. A fluid detecting device comprising: a main path through which afluid runs; one or a plurality of branch paths each having an openingend portion that is formed in the vicinity of an inner wall surface ofthe main path so as to point to an upstream or downstream side of saidmain path, and causing part of a fluid flowing in the vicinity of theinner wall surface of said main path to run therethrough via the openingend portions; and a thermal flow sensor that is disposed in said branchpath and detects a flow of the fluid running through said branch path.2. The fluid detecting device according to claim 1, wherein the openingend portions of said plurality of branch paths, which are pointed towardthe upstream side of said main path, are arranged at regular intervalsalong a path cross section of said main path around an axis of said mainpath.
 3. The fluid detecting device according to claim 1, wherein theopening end portions of said plurality of branch paths, which arepointed toward the downstream side of said main path, are arranged atregular intervals along a path cross section of said main path aroundthe axis of said main path.
 4. The fluid detecting device according toclaim 1, wherein said branch paths are open at the other end sidestoward a surrounding environment of said main path.
 5. The fluiddetecting device according to claim 2, wherein: the plurality of branchpaths whose opening end portions are pointed toward the upstream side ofsaid main path are connected to the other end portions to form one path;and said thermal flow sensor is disposed in a portion where saidplurality of branch paths are connected to one another to form one path,and detects a total flow rate of fluids running through the branchpaths.
 6. The fluid detecting device according to claim 3, wherein: theplurality of branch paths whose opening end portions are pointed towardthe downstream side of said main path are connected to the other endportions to form one path; and said thermal flow sensor is disposed in aportion where said plurality of branch paths are connected to oneanother to form one path, and detects the total flow rate of the fluidsrunning through the branch paths.
 7. The fluid detecting deviceaccording to claim 5, wherein an end portion of the portion where theplurality of branch paths are connected to one another to form one path,is open toward the surrounding environment of said main path.
 8. Thefluid detecting device according to claim 1, further comprising: anauxiliary thermal flow sensor that is disposed in a portion where saidbranch paths are not formed, and detects a state of said fluid.
 9. Thefluid detecting device according to claim 8, wherein said auxiliarythermal flow sensor is disposed in a fluid-pooling portion thatcommunicates with said branch paths.
 10. A fluid detecting devicecomprising: a main path through which a fluid runs; one or a pluralityof branch paths each having an inflow-side opening end portion pointedtoward an upstream side of said main path and an outflow-side openingend portion pointed toward a downstream side of said main path which areformed in the vicinity of an inner wall surface of the main path, andcausing part of a fluid flowing in the vicinity of the inner wallsurface of said main path to run therethrough via said inflow-side andoutflow-side opening end portions; and a thermal flow sensor that isdisposed in said branch path and detects a flow of the fluid runningthrough said branch path.
 11. The fluid detecting device according toclaim 10, wherein the inflow-side opening end portions and therespective outflow-side opening end portions in said plurality of branchpaths are arranged at regular intervals along a path cross section ofsaid main path around an axis of said main path.
 12. The fluid detectingdevice according to claim 10, wherein the inflow-side opening endportions and the respective outflow-side opening end portions in saidplurality of branch paths are roughly aligned in a path direction ofsaid main path.
 13. The fluid detecting device according to claim 11,wherein: said branch paths have a plurality of upstream-side branchpaths provided with said respective inflow-side opening end portions, aplurality of downstream-side branch paths provided with said respectiveoutflow-side opening end portions, and a communicating portion disposedbetween said plurality of upstream-side branch paths and said pluralityof downstream-side branch paths to form one path; and said thermal flowsensor is disposed in the communicating portion that forms said onepath, and detects a total flow rate of the fluids running through saidbranch paths.
 14. The fluid detecting device according to claim 13,wherein path resistance of each of said branch paths is greater thanpath resistance of said communicating portion.
 15. The fluid detectingdevice according to claim 10, further comprising an auxiliary thermalflow sensor that is disposed in a portion where said branch paths arenot formed and detects a state of said fluid.
 16. The fluid detectingdevice according to claim 15, wherein said auxiliary thermal flow sensoris disposed in a fluid-pooling portion that communicates with saidbranch paths.
 17. The fluid detecting device according to claim 2,wherein said branch paths are open at the other end sides toward asurrounding environment of said main path.
 18. The fluid detectingdevice according to claim 3, wherein said branch paths are open at theother end sides toward a surrounding environment of said main path. 19.The fluid detecting device according to claim 6, wherein an end portionof the portion where the plurality of branch paths are connected to oneanother to form one path, is open toward the surrounding environment ofsaid main path.
 20. The fluid detecting device according to claim 12,wherein: said branch paths have a plurality of upstream-side branchpaths provided with said respective inflow-side opening end portions, aplurality of downstream-side branch paths provided with said respectiveoutflow-side opening end portions, and a communicating portion disposedbetween said plurality of upstream-side branch paths and said pluralityof downstream-side branch paths to form one path; and said thermal flowsensor is disposed in the communicating portion that forms said onepath, and detects a total flow rate of the fluids running through saidbranch paths.
 21. The fluid detecting device according to claim 20,wherein path resistance of each of said branch paths is greater thanpath resistance of said communicating portion.